The assembly operation for integrated circuits, sometimes referred to as the "back-end", consists of taking a silicon wafer containing several hundred or more integrated circuit chips; testing the chips to determine which are working and which are not; cutting the chips apart; attaching the chip to a leadframe; bonding wires to the leads one at a time; and encapsulating the combination in plastic to provide protection for the device and cutting and bonding the external parts of the leadframe to their final shape.
The standard method of attaching wires to the chip is by wire bonding, in which a gold or aluminum alloy wire is pressed very hard (in the presence of elevated temperatures and/or ultronsonic energy) against a pad on the chip until a bond is formed. One wire is done at a time. This method uses a great deal of labor and expensive materials. Automated wire bonding machines are known, but they have an inherent limitation. Even with the most rapid machines that can be imagined, there are factors that provide a necessary limitation to approximately 2,000 units per hour for a 16 pin chip. With wire bonding techniques, it is also necessary to attach the chip to the package or leadframe in order to maintain it in position while the wire bonds are formed. Also, the leads in this process must be made of an expensive expansion-controlled alloy in order to have the correct thermal expansion matching between the chip and the leads or expensive special adhesives must be used to correct the thermal mismatch. Also, the leads must be plated with gold, silver or other precious metal so that the bending wire can form a reliable connection to the lead.
One prior art method that provides for simultaneous lead soldering is the "flip-chip" method developed by IBM, in which a lump of solder is placed on the chip and the chip is soldered to a ceramic substrate that is attached to the leads. This IBM method does not have a layer of leads on the top of the chip.